The present invention relates generally to methods for manufacturing contact lenses, and more particularly to methods for manufacturing finished aspheric single vision contact lenses, or finished spherical or aspheric bifocal, multifocal or progressive addition contact lens.
Contact lenses are generally manufactured by a machining or casting process. The machining process begins with a plastic disk or a blank, which is blocked on an arbor with a suitable wax, then mounted into a collet of a multi-axis lathe turning machine. After machining the first surface, the part is transfer blocked to a second side arbor and the second surface machined as before. Such a simple lathe turning process can only provide centrosymmetric optic geometries. Contact lenses embodying non-centrosymmetric geometries can only be manufactured by a casting process using a mold having an appropriate surface geometry. Thirty years of studying the casting process has led to the development of several efficient and rapid molding processes for contact lenses that incorporate complex optical geometries, such as toric contact lenses, bifocal aspheric contact lenses and so on.
While contact lenses are worn by over 10% of all antimetropes in the U.S., bifocal or multifocal contact lenses have enjoyed only a mixed success. No bifocal or multifocal contact lens has been successfully accepted by more than 70% of the patients fitted with a particular design. It is believed that a contact lens fits over the cornea of an individual in a unique manner, so that the spacial relationship between the optical center of the lens and the cornea is not entirely predictable. In other words, the lens undergoes microscopic movement after being fitted on a patient, until it achieves the more stable position over the cornea. This movement is too small to cause any significant change in the refractive correction provided by the lens, if the lens is of single vision type; however, for certain bifocal contact lenses to work properly, the add power zone must always line up within the pupillary aperture, therefore, even this microscopic repositioning over the cornea may shift the add power zone out of the pupillary aperture and create refractive problems for the patient.
The reason why the add power zone must be centered with respect to the pupil and occlude a certain optimum fraction of the pupillary aperture is that for a multifocal lens to function properly, the retina should receive all the images at the same time. For distant objects, the image formed by the base power zone is focused, while the image formed by the add power zone is not focused. For near objects, the image formed by the base power zone is defocused, while the image formed by the add power zone is focused. Given one focused and one or more defocused images, the image processing apparatus at the retina and the visual cortex rejects the unfocused images and processes the focused image.
Persons with normal accommodation not requiring any refractive correction also receive multiple images simultaneously at their retina, and possess the ability to ignore the defocused image of far objects when looking at near objects, and vise versa. This analogy to a normal eye indicates that for a bifocal or multifocal contact lens to work properly, the add power zone must be entirely within the pupillary aperture. Since image strength at the retina is proportional to the area of the corresponding refractive zone (i.e., add or base power) subtended at the pupil, the optimum area of the add power zone can be computed with respect to the pupil size. It is known that pupil size varies from person to person and also depends on the level of ambient illumination and physicochemical status of the individual. For example, the pupil size of a thirty year old can vary from 2.2 mm in direct sunlight to 5.7 mm outdoors at night. Data on pupil size distributions by age and illumination level are available in the literature. The assumption may also be made that the contact lens wearer will generally be outdoors when experiencing extreme levels of illumination, where distance vision will be needed the most, whereas ambient illumination is at an intermediate level indoors, where near and intermediate vision is required most often. Based on these considerations, it is possible to develop a model which predicts the optimum sizes of the add power zone for near vision/base power zone for distance vision and aspheric zones for intermediate vision, if needed. Such a model is disclosed in U.S. Pat. No. 5,112,351.
In some circumstances, it is also useful to properly position the toric surface of a contact lens with respect to the pupil of an astigmatic user. A more significant problem associated with toric or aspheric lenses is the sheer number of lenses that must be kept in stock for adequate customer selection. For example, to stock all of the possible combinations for lenses over a spherical range of -15.00 D to +15.00 D in 0.25 D steps (121 spherical powers), over a cylinder range of 0.00 D to -5.00 D in 0.25 D steps (21 powers), and an astigmatic rotation range of 180.degree. in 2.degree. steps (90 rotations) would require that 228,690 lenses be stocked. Even if the stigmatic range is broken down into 15.degree. steps (12 rotators), as is commonly done in lens manufacturing, 30492 lenses would have to be stocked. Ideally, the practitioner would only stock a set of lenses having the required sphere and cylinder, while later correcting for rotation (2541 lenses), or stock a set of spherical lens, while later adding a toric surface for cylinder and adjusting for rotation (121 lenses), or stock a set of toric lenses while later correcting for sphere or rotation (21 lens), or stock a set of preforms having a single power, while later correcting for sphere, cylinder and rotation.
In view of the above, there is a need for a contact lens where the add power zone or toric zone is precisely positioned with respect to the pupil of the wearer, and for a process for making the same. At the same time, there is also a need for lenses that can be easily modified in-house to provide proper sphere, cylinder and rotation as needed.